The present invention relates to a dryer section of a paper machine including cylinder groups provided with single-wire draw which comprise a plurality of heated drying cylinders having smooth faces against which a paper web to be dried enters into direct contact, and reversing rolls. The single-wire draw groups have a drying wire which is guided in a loop by guide rolls as well as by the drying cylinders and reversing rolls such that the drying cylinders are arranged outside the wire loop, and the reversing rolls are arranged in gaps between the drying cylinders and inside the drying-wire loop. The present invention also relates to a method for drying a web in a dryer section.
As known in the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed in dryer groups. In a twin-wire draw dryer group, there are two wires which press the web, one from above and the other one from below, against the heated cylinder faces of the drying cylinders. In twin-wire draw, between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering leading to web breaks, in particular in the stages of the drying in which the web is still relatively moist and, therefore, of low strength. For this reason, in recent years, increasing use has been made of the single-wire draw, in which each group of drying cylinders has only one drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas, on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop and the reversing cylinders or rolls are situated inside the wire loop. From the prior art, dryer sections are known that consist exclusively of so-called normal groups with single-wire draw, in which groups the drying cylinders are placed in a first, upper row and the reversing cylinders or rolls are placed in a second, lower row below the first row.
In a dryer section that consists of normal groups with single-wire draw alone, it is a drawback that the paper web is dried from the side of its lower face alone, which may cause a tendency of curling in the web. Therefore, it is known in the prior art when drying a paper web, to use a dryer section in which there are, for example, alternatingly so-called normal groups and inverted groups with single-wire draw. In an inverted group with single-wire draw, the drying cylinders are situated in a first, lower row and the reversing cylinders or rolls are situated in a second, upper row above the first row of drying cylinders.
Also, dryer sections are known in which all the other groups except the last group are normal groups with single-wire draw, whereas the last group is a group with twin-wire draw.
In the dryer sections mentioned above, various problems have occurred, for which problems the present invention provides novel and efficient solutions. These problems include the large length of the dryer section, which increases the costs of the dryer section and of the machine hall. Problems have also been encountered in the runnability of the dryer section and in the threading of the web, as well as those problems which arise from differences in the speeds of different wires and problems related to the control of transverse shrinkage of the web. Generally, these problems tend to become worse as the running speed of the paper machine becomes higher.
In prior art dryer-section constructions, the overall length of the dryer section is often considerably large, which results in an increase in the cost of investment of a paper mill, above all the construction cost. If it were possible to reduce the length of the dryer section, it would also be possible to reduce the construction cost of the paper mill in the same proportion.
Regarding the prior art related to the present invention, reference is made initially to Finnish Patent Application No. 913648 (corresponding to U.S. Pat. No. 5,279,050, the specification of which is hereby incorporated by reference herein). In the dryer section described in this Finnish patent application, drying cylinders of two different diameters and reversing cylinders of two different diameters are used, of which cylinders the drying cylinders with larger diameter are placed in an uppermost first, horizontal level, and a reversing cylinder is placed in each gap between every other pair of upper drying cylinders and the drying cylinders of smaller diameter, placed in gaps between every other pair of cylinders, are situated in the next, lower second horizontal level. The last-mentioned reversing cylinders and drying cylinders have substantially equal diameters. In the lowest, third level, reversing cylinders of smaller diameter are placed in pairs at both sides of the gap between every other pair of upper drying cylinders, a drying cylinder of smaller diameter being placed above, and in the middle of, the gap between the pair of lower reversing cylinders. A drawback of the dryer section of this patent application is that the drying cylinders and the reversing cylinders are placed at quite a large difference in height in relation to one another, so that servicing of the machine from one tending platform is difficult, and that, for the machine, both drying cylinders of two different diameters and reversing cylinders of two different diameters are needed. This increases the costs of manufacture, servicing and spare parts of the dryer section.
With respect to the prior art, reference is also made to Finnish Patent No. 83,441, in which it is suggested that, over the length of the dryer section, different wire tensions be employed in the different groups, preferably so that, when the dry solids content becomes higher, the wire tension is increased. However, in this manner, the drying capacity cannot be influenced so efficiently, and a further drawback of this technique is that large-diameter guide rolls are required in order to obtain high wire tensions. The concept of this Finnish patent can, however, be used favorably in connection with the present invention.
With respect to the prior art closely related to the present invention, reference is made further to U.S. Pat. Nos. 4,882,855, 4,982,513 and 5,241,761 assigned to J. M. Voith GmbH). Of these patents, U.S. Pat. No. 4,882,855 describes a dryer section in which there are, in the direction of progress of the web, initially drying cylinders of constantly increasing diameters, after which there are, in the direction of progress of the web, drying cylinders having a constantly decreasing diameters. It is a drawback of this dryer section construction that the costs of manufacture, servicing and spare parts of the dryer section become very high because of the number of different diameters of the drying cylinders. A further drawback is that there are open draws of the web between the rows of drying cylinders.
In U.S. Pat. No. 4,982,513, a dryer section is described which consists of a number of successive groups with single-wire draw, which groups are alternatingly so-called normal and inverted groups. In the initial end of the dryer section, obviously drying cylinders of normal size are used, for example, in the first two groups, and thereafter, in the following two groups, cylinders larger than the normal size are used. Then, in the following wire groups, in the direction of progress of the web, drying cylinders of successively smaller diameters are used. The reversing suction rolls placed between the drying cylinders are rolls of equal diameter in all the groups. It is a drawback of this construction that cylinders of at least four different diameters are used, and that the cylinder diameters are not arranged so that they are distributed optimally in view of the drying capacity in the direction of progress of the web.
In U.S. Pat. No. 5,241,761, a dryer section is described which consists of a number of successive groups with single-wire draw arranged in such a way that, in the initial end of the dryer section, in the single-wire groups, small-diameter reversing suction rolls with an inside or interior suction box are used, and in the wire groups in the rear end of the dryer section, cylinders of larger diameter and with grooved mantles are used. These large-diameter grooved-mantle cylinders resemble the suction cylinders which are marketed under the trade mark "VAC-ROLL".TM., which have no inside suction boxes, and which are described in Finnish Patent No. 83,680 (corresponding U.S. Pat. No. 5,022,163, the specification of which is hereby incorporated by reference herein). In the dryer section described in U.S. Pat. No. 5,241,761, drying cylinders of just one diameter are employed.